About 50% of agricultural accident fatalities occur in tractor rollovers. The end can come quickly for an unsuspecting farmer as the time from “point of no return”, when rollover is inevitable, to impact takes less than one second. D. J. Murphy et al. (Applied Ergonomics 1985, 16.3, 187-191) describe a plausible rollover scenario:
“A tractor operator is baling hay with a large round baler. He is operating the tractor at 4 mph on ground that starts out with a 5% slope at the outer edges of the field. The windrows on the outside of the field are sufficiently rounded, so he hasn't had to slow down to make turns. But as he moves toward the center of the field, the slope has gradually increased to 20% and the turns have grown slightly tighter.”
“He hasn't cut his speed yet, but he has noticed that there is less time to correct his steering after the turn to stay on the windrow. On the next round, a narrow rise, where the inside rear tire travels, raises the slope to 23% and the turning angle is tightened once again. Unbeknownst to the operator, the slight increase in slope, decrease in radius, and constant speed put his tractor right on the brink of overturn.”
“As he starts into the turn of this new round, the operator, from previous experiences, senses that he needs to slow down. But just as he reaches for the throttle, his eye catches sight of a groundhog hole that the front wheel is about to drop into. The presentation of this new bit of stimuli causes the slightest hesitation as it is transmitted to the brain and analyzed. Almost instinctively, the operator quickly yanks the steering wheel tighter to avoid the hole. This final act results in the tractor rolling over.”
Tractor stability is a widely studied topic because of its importance to farm safety. Static stability refers to the effect of tractor attitude (pitch, roll, and yaw) and the projection of the center of gravity inside or outside a stability baseline. Dynamic stability takes into account effects of motion, speed and turning maneuvers.
FIG. 1 shows a rear view of a tractor 105 on slope 110. The position of the tractor's center of gravity (CG) 115 is marked by a circle with a cross inscribed in it. Arrow 120 is drawn along a vertical line coincident with the center of gravity; i.e. it shows the direction from the center of gravity to the center of the earth. In FIG. 1 arrow 120 crosses the slope just inside the tractor's rear wheel. If the tractor were to tip such that the arrow lay outside the wheel, the tractor would roll over in the direction of curved arrow 125.
FIG. 2 is a tractor stability baseline diagram. The diagram shows a top view of a tractor's wheels (200, 205) and center of gravity 210. Dotted trapezoid 215 connects points at the outside, center of each wheel where the wheel touches the ground. This trapezoid is called the stability baseline. If a line (such as arrow 120 of FIG. 1) drawn from the center of gravity toward the center of the earth passes within the stability baseline, the tractor is stable. On the other hand if the line passes outside the baseline, the tractor is unstable and will roll over.
Rollovers may occur to the side, rear or front. Most often, side rollovers are the result of driving on too steep a slope while rear rollovers are caused by trying to pull an object with a hitch point located too high on the tractor. Front rollovers are rare. However, rollovers may occur for any number of reasons that contribute to an accident chain.
FIG. 3 lists common hazards that increase rollover risk. These hazards include driving on too steep a slope and encounters with bumps or ditches. Some maneuvers, such as uphill turns, are safe at slow speed, yet pose significant rollover risk at higher speeds. Sharp turns increase risk compared to gradual turns. The position of a tractor's center of gravity affects rollover risk greatly. High center of gravity conditions caused by unusual loads (e.g. spray tanks) or lifted implements (e.g. buckets) increase rollover risk. Driving with a flat tire can make an otherwise tolerable slope traverse impassable.
A common theme among rollover hazards is that it is not always easy for a tractor operator to perceive the level of rollover risk associated with a particular maneuver. Driving along a particular path may be safe sometimes and dangerous at other times depending on speed, center of gravity location, type of tractor and other factors. Therefore, what are needed are systems and methods for warning tractor operators of rollover hazards as far in advance as possible.